1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer, direct digital plate-making machine or similar electrophotographic image forming apparatus and more particularly to a developing device using a magnetic force and a process cartridge including the same.
2. Description of the Background Art
Generally, in an electrophotographic image forming apparatus, a latent image is formed on an image carrier in accordance with image data and then developed by a developing device to become a toner image. It is a common practice with this type of image forming apparatus to use a two-ingredient type developer made up of nonmagnetic toner grains and magnetic carrier grains.
In a developing system using a two-ingredient type developer, the shorter the distance between the image carrier and the developer carrier in a developing zone, the more adequate the image density and the less the edge effect, as known in the art. Also, to enhance the developing ability and therefore image density, the amount of developer to be fed may be increased to increase the amount of developer in the developing zone. However, these schemes both bring about carrier deposition. Carrier deposition refers to a phenomenon that an electric force derived from an electric field between the carrier grains and the image carrier overcomes a magnetic force exerted on the carrier grains by the developer carrier and prevents the magnetic force from returning the carrier grains around the image carrier toward the developer carrier.
To obviate carrier deposition, the charge potential of the image carrier and the potential of the developer carrier may be so controlled as to reduce the electric force exerted by the image carrier. This, however, gives rise to another problem that the toner grains are apt to deposit on the non-image portion or background of the image carrier and contaminate it.
Today, the grain size of carrier and that of toner are decreasing in order to meet the increasing demand for higher image quality. Although reducing the grain sizes of carrier and toner enhances image quality, as reported in the past, this scheme aggravates carrier deposition. This is particularly true when the grain size of carrier is reduced.